2) Monodepth2+USIP converts the cross-modality registration problem into point cloud-based registration by using Monodepth2 [1]} to estimate a depth map from a single image. The Lidar point cloud is used to calibrate the scale of depth map from MonoDepth2, i.e. the scale of the depth map is perfect. Subsequently, the poses between the depth map and point cloud are estimated with USIP [2]}. This is akin to same modality point cloud-to-point cloud registration. Nonetheless, Table REF  shows that this approach under-performs.
This is probably because the depth map is inaccurate and USIP does not generalize well on depth maps.
